1. Field of The Invention
The invention relates to a scanning device for optically scanning surface of an object along a line, which device comprises a radiation source unit for supplying a scanning beam, an optical system for forming a point of smallest constriction in the scanning beam and for forming a scanning spot on the surface by means of the scanning beam, and a deflection system for deflecting the scanning beam through a variable deflection angle to a position on the surface to be scanned, said optical system comprising a main imaging system which is arranged in the radiation path of the scanning beam between the radiation source unit and the deflection system and a correction system which is arranged in the radiation path of the scanning beam between the deflection system and the surface to be scanned.
2. Description of the Related Art
A scanning device of the type referred to above is described by K. Klose in the Article "Application of additional mirrors for rectilinear laser scanning of wide formats", published in "Applied Optics", Vol. 17, No. 2, pp. 203-210 (1978). As has been described in this article, the point of smallest constriction in the scanning beam follows a curved track in the absence of the correction system. The point of smallest constriction is hereinafter referred to as the image point. Therefore, a correction system for converting this curved track to the straight line on the surface is required for high-resolution scanning of a flat surface along a straight line. In the known device this is effected in that the scanning beam is reflected by means of the correction system towards the line to be scanned in such a way that the total optical path length traversed by the radiation beam from the objective system to the line to be scanned remains equal to the image length, i.e. the distance between the objective system and the image point.
In the known device the focusing action of the correction system used is ignored. This focusing action results in the focal length of the entire optical system being smaller than that of the objective system alone. Moreover, the optical power of the optical system varies when the different parts of the correction system are traversed, and thus varies as a function of the adjustment of the deflection system. Also the position occupied by the correction system in the scanning beam varies. This means that the image point, the narrowest point in the scanning beam, is present at some distance from the line to be scanned, which distance also varies as a function of the magnitude of the deflection angle.
The known correction mirror is curved in one direction only so that the extra focusing effect also occurs in one direction only. A consequence of the resulting astigmatism is that the focusing effect cannot be compensated for by a displacement towards the correction system of the surface to be scanned. Due to the above-mentioned variation in the optical power of the optical system, this astigmatism cannot be simply corrected, for example by means of a cylindrical lens.
Due to the reversibility of the radiation path an optical system in a device as described in the opening paragraph may alternatively be used for scanning a surface from which radiation originates. The radiation from an area on the surface is then concentrated on a radiation-sensitive detection system by means of the optical system. The invention thus also relates to a scanning device for optically scanning the surface of an object along a line, which device comprises a radiation-sensitive detection system for detecting radiation from the surface, an optical system for imaging an area of the surface on the radiation-sensitive detection system and a deflection system for selecting said area of the surface, said optical system comprising a main imaging system which is arranged in the radiation path between the deflection system and the radiation-sensitive detection system and a correction system which is arranged between the surface to be scanned and the deflection system. Also if the scanning device comprises a radiation-sensitive detection system, the area to be scanned by a scanning device which is provided with the known optical system is imaged with a varying extent of defocus on the radiation-sensitive detection system. The invention also relates to a correction system which is suitable for use in such scanning devices.